Our ultimate goal is the complete understanding of neural mechanisms for both species-specific (biosonar) and common (communication) auditory functions. Unlike the auditory periphery, the central auditory system contains many different types of neurons. Some neurons are tuned to particular acoustic parameters characterizing sounds other than frequency. Some of these, called "combination-sensitive" neurons, are tuned to specific parameters characterizing combinations of signal elements in complex sounds. Different types of neurons are clustered in different cortical areas. It has been explained that all these neurons result from the divergent and convergent projections within the ascending auditory system. However, the descending (corticofugal) system plays a very important role in signal processing. Response properties of subcortical neurons, and accordingly those of cortical neurons, are shaped by both the ascending and descending systems. The organization of the subcortical nucleus can be changed by the corticofugal system according to auditory experience, including associative learning. Our aims for the proposed research are to explore the functions of the corticofugal system and to test our hypotheses: (1) that the auditory cortex has an intrinsic mechanism which works together with the corticofugal system to adjust and improve auditory signal processing according to auditory experience, (2) that this mechanism is augmented if the acoustic signals become behaviorally relevant to the animal, e.g., through associative learning, and (3) that such augmentation is mediated by the cholinergic basal forebrain. Our proposed research will contribute further toward our understanding of the neural mechanisms for processing behaviorally relevant sounds and the functional organization of the central auditory system. We will be able to propose possible neural mechanisms for hearing disorders such as neural tinnitus. We will study the response properties of cortical and subcortical neurons to acoustic stimuli. We will then study how their response properties are changed by focal electrical stimulation of the auditory cortex, somatosensory cortex and/or basal forebrain or classical conditioning and also by focal applications of different types of drugs to the auditory cortex and subcortical nuclei. All the data which will be obtained will be evaluated in relation to our hypotheses.